The invention relates to a device for optically measuring the height of a surface irregularities of an object along an axis substantially normal to such surface, which device comprises a radiation source for generating a radiation beam, means for forming the beam into a radiation spot on the surface of the object, an imaging system for forming a reflected or transmitted image of the scanning spot in a plane, and a radiation detection system for determining the position of the imaged spot in said plane.
A device of this type is used, for example, to check by means of triangulation whether a surface has the desired profile, for example, whether a flat plate is really flat and does not have any grooves or projections. The device may also be used to check whether holes and elevations are provided at the correct positions on a substrate plate, and the device may be further used, for example, for inspecting electronic circuits comprising electrically insulating plates provided with conducting metal strips and electronic components.
A device as described in the opening paragraph is known, inter alia, from published application EP-A 0,134,597. This document describes a device for performing a triangulation so as to determine the distance between the surface of an object and a reference level. The known device comprises a radiation source from which a narrow radiation beam is incident on the surface where it forms a radiation spot. The reflected image of the radiation spot is focused on a linear radiation-sensitive detector surface by means of one or more objective lenses. The position at which the imaged radiation spot is located on the detector surface corresponds to where the radiation beam the impinged upon the object surface and hence indicates the height of the latter surface. A displacement of the position of the radiation spot across the detector surface thereby enables changes in height of the surface of the object to be measured.
The sensitivity of the height measurement, i.e. the ratio between the change of the surface height and the displacement of the position of the incident radiation spot across the detector surface is, inter alia, dependent on the degree of tilt of the surface of the position-sensitive radiation detection system with respect to the incident radiation beam or the optical axis of the imaging system. However, if a small angel is chosen in order to obtain a maximum sensitivity, with the radiation grazing the detector surface, a very large part of the radiation will be reflected by the detector surface and only a small fraction can be measured. Utilization of the tilt of the detector surface to enhance the sensitivity of the height measurement is thus restricted.